Research on a massive MIMO technique wherein data is transmitted to a plurality of User Equipments (UEs) using a MIMO (i.e., a multi-user MIMO) technique is progressing as a core communication technique in a next generation wireless communication system. Theoretically, since a transmission beamforming gain is proportional to the number of transmission antennas, when a high beamforming gain of a massive MIMO system is used, each transmission antenna comes to the forefront as an important technique of green communication because each transmission antenna uses low transmission power.
As a cell of a mobile communication system becomes smaller and the number of transmission antennas of an eNodeB increases, a capacity gain by a beamforming, an SM and an SDMA in a vertical dimension in addition to the existing horizontal plane (i.e., a horizontal dimension) is expected. Therefore, a necessity of a research for increasing a capacity in the massive MIMO system using the two-dimensional planar array antenna has emerged.
Thus, a development of a technique for effectively transmitting a Channel State Information Reference Signal (CSI-RS) for a downlink two-dimensional space (i.e., horizontal and vertical dimensions) channel estimate by using only a limited amount of Resource Elements (REs) and for feeding Channel State Information (CSI) using only a limited feedback is necessary. In addition, a development of a hybrid SM & SDMA technique which optimally selects and performs the SM and the SDMA in the two-dimensional space channel using this is necessary.